Most polymers are known to degrade upon prolonged exposure to sunlight or other forms of light. One of the effects observed is yellowing of the polymer or its blends. This problem has been partially alleviated by the use of light absorbers or light blockers which restrict the penetration of light and thus minimize the degradation to the polymer surface. Thus ultraviolet light absorbers (UVA) are known to be used to protect polymers from degradation due to exposure to different sources of light.
Use of UVA's, however, does not offer complete protection as degradation still occurs at the surface and ultimately leads to discoloration and surface embrittlement, albeit at a reduced rate. The life time of many polymers in a weathering environment may be prolonged by increasing the UVA loading, however a practical limit is eventually reached above which the physical properties of the polymer begin to deteriorate with further addition of UVA's. While high loading of UVA's may offer enhanced protection, depending on the application, they can also result in high initial color.
One strategy to protect light induced polymer degradation involves the use of latent UVA's in the form of aromatic polyarylates. When exposed to ultraviolet light, these compositions undergo the photo-Fries rearrangement to generate substituted 2-hydroxybenzophenone moieties which act as UV absorbers as reported by S. B. Maerov in J. Polym. Sci. Part A, Vol. 3, pp. 487-499, 1965, and protect the compositions from further degradation. These polyarylates generally are based on esters of bis phenols, such as bisphenol A, with isophthalic acid and terephthalic acid and have outstanding retention of physical properties upon weathering. An example of such a polyarylate is Ardel.RTM. D-100 available from Union Carbide. However, the 2-hydroxybenzophenone moieties generated from such polymers are very yellow and are undesirable in applications that require little change in color during weathering.
Attempts have been made to avoid color formation by substituting resorcinol for the bis phenol unit in the polyarylates, as reported by S. M. Cohen, R. H. Young, and A. H. Markhart in J. Polym. Sci. Part A-1, Vol. 9, pp. 3263-3299, 1971. While using resorcinol results in much lower color upon irradiation, polymers based on poly(resorcinol terephthalate) or poly(resorcinol isophthalate) are reported to have poor solubility and are difficult to synthesize to high molecular weight by conventional interfacial or solution processes due to the formation of highly crystalline oligomers. On addition, the glass transition temperatures of such compositions need to be high, preferably greater than about 100.degree. C., so that the polymers can be used in engineering thermoplastic applications. U.S. Pat. No. 4,390,683 discloses a monoaxially or biaxially stretched film structure, or a heat-set structure thereof, for an unstretched film of an aromatic polyester of the poly-1,3-phenylene terephthalate type. This patent, however, does not report the initial color or weatherability of the stretched film structures.
There is thus a need for high molecular weight polyarylate polymer compositions which can be easily prepared in high yield, and exhibit little change in color upon irradiation. There is also a continued need for polymers that when exposed to light do not lose the desirable physical properties of the polymer.